(1) Field of the Invention
The present invention generally relates to an antilock braking control apparatus, and more particularly to an antilock braking control apparatus for an electric vehicle in which both a regenerative braking system and a friction braking system are used.
(2) Description of the Related Art
An electric vehicle is propelled by a traction motor using electric power supplied from a storage battery, and wheels of the electric vehicle are rotated by the traction motor. It is expected that the use of electric vehicles will become increasingly common in the future since they will not cause the environmental pollution which is the dominant problem of gasoline-fuel vehicles.
Generally, the wheels of the electric vehicle are braked by using a combination of an electrical braking system and a mechanical braking system. It is difficult to reliably brake the wheels of the electric vehicle by using the electrical braking system only.
Hereinafter, the electrical braking refers to a regenerative braking method or system by which the kinetic energy of the vehicle derived from the traction motor is transformed into electric energy, and the electric energy is re-stored in the storage battery. The braking of the traction motor is achieved by the regenerative braking system, and the wheels of the vehicle are thus braked.
Hereinafter, the mechanical braking refers to a known friction braking method or system which is commonly used in gasoline-fuel automotive vehicles. For example, a hydraulic brake is used as one of the mechanical braking systems. Generally, the gasoline-fuel vehicle has a plurality of hydraulic brakes operatively connected with the wheels of the vehicle, and the wheels are braked by the wheel-to-road friction produced by the hydraulic brakes by using the brake fluid pressure actuated to the hydraulic brakes.
An antilock brake system (ABS) is known as a braking system of an automotive vehicle. The ABS is designed for preventing braked wheels from slipping or skidding if the brake torque exceeds the available wheel torque capacity, and the braking force acting on the wheels is effectively reduced by the ABS especially when the vehicle is running on a snowy or iced road and the wheels are suddenly braked.
Japanese Laid-Open Patent Publication No. 2-141354 discloses an antilock brake control device for an electric vehicle. The electric vehicle uses an electric brake and a pneumatic brake for the braking of the wheels, and the antilock brake control device performs an ABS control process when the slippage of the braked wheels is detected. During the execution of the ABS control process, the regenerative braking force of the electrical brake is reduced, and at the same time the braking force by the pneumatic brake is held at a given level for a given time period.
Therefore, in the conventional device disclosed in the above mentioned publication, the braking force by the pneumatic brake is acting on the related wheel when the regenerative braking force by the electric brake is reduced during the execution of the ABS control process. The effect of the ABS control process to prevent the braked wheels from slipping on the road is influenced by the pneumatic braking force held at the given level. For this reason, the running stability and controllability of the vehicle is likely to be worse when the ABS control process is performed.